Inkjet inks used for inkjet recording system are generally classified into aqueous inks and non-aqueous inks. Non-aqueous inks are generally classified into solvent inks composed mainly of volatile organic solvents and oil inks composed mainly of non-volatile organic solvents.
Of these, oil inks are suitable for high speed inkjet printing because the solvent is difficult to volatilize, and thus clogging in ink nozzles is unlikely to occur, thereby reducing frequency of cleaning of ink nozzles (refers to Patent Document 1 and Patent Document 2).
However, when plain paper is used as a printing medium, the oil ink is disadvantageous in that the solvent is easy to permeate into gaps between fibers of the printing paper, and consequently coloring materials are difficult to stay on the surface of the printing medium, thereby lowering printing density and causing strike through.
Conventionally, it has been proposed for an aqueous ink that printing density is improved by ejecting a cationic ink onto a site where an anionic ink has been previously ejected, whereby acid-base reaction is caused to insolubilize or aggregate coloring materials contained in these inks (refers to Patent Document 3 and Patent Document 4).    [Patent Document 1] JP-A-2003-261808    [Patent Document 2] JP-A-2007-126564    [Patent Document 3] JP-A-H8-281930    [Patent Document 4] JP-A-2004-338361